docs: def tags(self, tags=None):
          """Filter by tags.
  
          :param  tags: Tags to filter.
          :type   tags: ``list``
  
          :return: A list of Node objects.
          :rtype:  ``list`` of :class:`Node`
          """
          if tags is None or not tags:
              return self
          nodes = []
          for node in self.nodes:
              if any(tag in node.extra['tags'] for tag in tags):
                  nodes.append(node)
          self.nodes = nodes
          return self